Polyphase power inverter system

ABSTRACT

Polyphase inverter system utilizing power silicon-controlled rectifiers. A plurality of polyphase square wave generators or channels are interconnected, each generator in a relative predetermined time phase displacement. Their respective outputs are herein arranged to directly provide multistepped waveforms, filtered to become the system polyphase power output. Each such waveform is composed of a significant number of steps whereby substantially reduced filtering is required. The generator channels are coupled in pairs, to respective delta and wye transformer arrangements, with their corresponding secondary windings in series-add summation, on a phase by phase basis. Reduced weight, bulk, and cost factors are significant, particularly for the summation transformers. Applicable for generating precise output frequency supply as 400-cycle polyphase aircraft installations, and for 60-cycle uninterrupted power ground stations.

United States Patent Ve Nard,II v

POLYPHASE POWER INVERTER SYSTEM Inventor: Dan L. Ve Nard, II, BonnersFerry, ldaho Assignee: Gates Learjet Corporation, Wichita, Kans.

Notice: The portion of the term of this patent subsequent to Nov. 4,1986, has been disclaimed. a

Filed: Oct. 21, 1969 Appl. No.: 868,190

Related [1.8. Application Data Continuation of Ser. No.630,061, Nov. 4,1969, Pat. No. 3,477,010.

US. Cl. ..32l/5, 321/9 R, 321/27 MS,

Int. Cl .....H02m l/l2, HOZm 7/52 Field of Search ..321/5, 9, 27'SW,DIG. 1

References Cited UNITED STATES PATENTS.

3,477,010 11/1969 Ve Nard ..321/27 X 7/1970 Anderson et a1 ..32l/27 X 3dscR-z 51 *Jan. 25, 1972 3,523,236 8/1970 Howell et al ..32l/9 3,391,3237/1968 lkeda 3,324,374 6/1967 Corey ..321/5 Primary ExaminerWilliam H.Beha, Jr. I Attorney-Richard A. Marsen s71 ABSTRACT Polyphase invertersystem utilizing power silicon-controlled rectifiers. A plurality ofpolyphase square wave generators or channels are interconnected, eachgenerator in a relative predetermined time phase displacement. Theirrespective outputs are herein arranged todireetly provide multisteppedwaveforms, filtered to become the system polyphase power output. Eachsuch waveform is composed of a significant number of steps wherebysubstantially reduced filtering is required. The generator channels arecoupled in pairs, to

. respective delta and wye transformer arrangements, with theircorresponding secondary windings in series-add summation, on a phase byphase basis. Reduced weight, bulk, and cost factors are significant,particularly for the summation transformers. Applicable for generatingprecise output frequency supply as 400-cycle polyphase aircraftinstallations, and for 60-cycle uninterrupted power ground stations.

11 Claims, 8 Drawing Figures PATENTED JAI|25|97Z I 3,638,094 m 1 or aSCR-l -souARE WAVE .1 CHANNEL A GENERATOR 53 --1 55 0 1 I l 1 tSEQUENTIAL 4 f v "rmcssnmc CIRCUITS 4o 45 54 sflscfl-z I 53 SQUARE WAVE:CHANNEL 2 G EN ERATOR -o FIG I I 1/ 56 )1 k OUTPUT 1 's TWO commas POWERmvzm'en 47+ 48 I FILTER 42 25 SCR! '7'". I 53 32$ SCR-2 INVENTOR, FIG 2DAN L.VENARD1I ATTORNEY.

PATENTED JANZS I972 sum 2 0r 5 PIC-3.4 SCR connucnon TO PRIMARY or sT-l(3p) F IG.5

OUTPUT OF SECONDARY(ST'|) LINE'TO-N 1 1 F IG. 6

OUTPUT OF SECONDARY (ST-l) -To-LmE A 72 PATENTEU Ma 51912 sheet 3 ur 5my ml 0 Tbmy FIG.7

ST-l

A 3;! PRIMARY WINDING SECONDARY VOLTAGE WAVESHAPE (SUMMING TRANSFORMERST-I) M US Q F\ w l- I w. I lawi nw m e SECONDARY VOLTAGE WAVESHA PE 2 B3g! PRIMARY wmome (SUMMING TRANSFORMER ST-Z) 3 n q s a m T. m 6 6 s N 2S e m R A D N o c E 8 n 1 W a .II C 6 OPTIMUM l2 STEP vuvasaap: e

D FUNDEMENTAL WAVE e AFTER FILTERING WAVESHAPE 8 POLYPIIASE POWERINVERTER SYSTEM The present invention is an improvement andcontinuationin-part of my copending application Ser. No. 630,061, thatissued on Nov. 4, 1969 as US. Pat. No. 3,477,101, assigned to theassignee thereof.

- As disclosed in said prior case, individual polyphase signal agenerators are utilized as building blocks to provide the rated powerrequirement for the inverter system. They are respec tively energized byadirect current source, and produce currents of square waveformalternating at a cyclic rate that predetermines the output frequency.The increasing power ratings of availablesilicon-controlled rectifiershas expanded polyphase inverter-applications to higherpowerinstallations. The plural interconnected square-wave generatorshereof make significant power outputs practical. Y

Rather complex and inefficient arrangements were heretofore utilized,that required relatively large and heavy and costly filter inductors andcapacitators. Further, they produced significant radio and magneticradiation interference, were rather unreliable, and were heavy for.airborne use.

a In accordance with the present invention, square wave polyphasegenerators orchann'els are interconnected inpairs, with respective deltaand wye output transformers. Efficient multistep polyphase waveforms areprovided directly in the system output. The paired generator channelsare respectively time-phase displaced in predetermined degree. Thepaired delta and wye-connected output transformer configurations hereofprovide the advantageous respective waveforms, which as a result oftheir said'time-phase displacement, mesh into composite multisteppolyphase waveshapes. The respective stepped signal summations require aminimum of filtering, and

relation, on a phase-by-phase basis between the channel pair's,

providing the desired multistep waveforms for the polyphase systemoutput. The delta andiwye transformers are of conventional polyphasetypes, compared to specialized types often required heretofore.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram of theexemplary polyphase inverter system.

FIG. 2 is a schematic diagram of the paired channel delta and wyetransformer interconnection, per the invention system.

FIG. 3 is a schematic electrical diagram of a three-phase square wavegenerator-channel for the exemplary system.

FIG. 4 illustrates the square waveform voltage output of the generatorof FIG. 3.

FIGS. 5 and 6 illustrate the waveform of respectivesignals appearing atthe'secondary windings of the wye-connected transformer.

FIG. 7 illustrates, in its sections A to D, the output waveformcomposite construction of the paired delta and wye channels per FIG. 2.

FIG. 8 illustrates the relative time-phase relationship of the squarewave outputs of the channel pair of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a schematic diagram of athree-phase inverter (40) hereof, arranged in two channels. Channel No.1 comprises square wave generator 55 that feeds into an associatedoutput or summing transformer ST-l. Channel No. 2 comprises square wavegenerator 56 that feeds into its transformer ST-2. Both square wavegenerators 55, 56 are solid-state threephase devices. They incorporaterespective banks of siliconcontrolled rectifiers arranged to provideproperly timed and phase voltages of sizable magnitude to transformersST-l and ST-2. Operationof the SCR banks is effected by sequentialtriggering circuits 54, which in turn are controlled by basefrequencyimpulses (1],) from unit 45. The triggering u'nit 54 is shown connectedby cables 53, 53' to the generators 55,56. Exemplary circuits for units55 and 56 are shown and described in connection with FIGS. 2 and 3.

The square wave generators (55,56) are powered by the direct currentlines (47,48) from a suitable DC source. Their three-phase square waveoutputs are respectively applied to the primary windings: of transformerST-I via leads S7; and of transformer ST-2 via leads 58. Thetransformers ST-I and ST-2 each are of conventional three-phaseconfiguration, and of stationary laminated iron construction. Thesecondary windings of output transformers ST-I and ST-Z areinterconnected, in series-add, providing optimum multistepped waveformsat the setsystem output frequency (1",). Such connection isindicated bymultiwire cable 70 between transformers ST-l and ST-2 with optionalfourth-wire neutral (N).

The summing transformers hereof (ST-1 ST-2), their electricalinterrelation with each other and with the signals and circuits of theinverters, are significant and'important to the effectiveness,flexibility, and efficiency of the present invention.

The exemplary inverter system (40) is a two-channelthreephaseconfiguration. The power inverters hereof, in their more generalaspects, may contain any practical even number of channels,-asmultichannel inverters. Also, the number of electrical phases thereofisoptional, as one-, two-, threeor sixphase output. The polyphasemultichannel inverters provide improved efficiency and higher powerratings. A circuit representation of the reference summing transformerST-l of channel No. l is shown in FIG. 2. It has a laminated iron core61-1 with a conventional polyphase physical configuration. Its primary60-1 comprises three-phase windings 62-1,63-'1,64- l woundinconventional array, interphase coupled magnetically by core 61-1. Thesecondary 65-1 comprises individual three-phase windings 66,67,68 woundin space-phase with the respective primary windings 62-1 ,63-I,64-l. Thesecondary windings are preferablywound close toand upon their respectiveprimary windings. The ST-l transformer primary and secondary 60-1 ,65-1thus comprises three phases: 45,4,

The transformer ST-2 for channel No. 2 is connected in deltaconfiguration at its primary 60-2. Also, importantly as well thetime-phase of the square wave'iriput from generator SCR-Z (56) to therespective primary windings 62-2,63- 2,64-4 are displaced in apredetermined amount. For the twochannel embodiment40, channel No. 2signals are time-displaced 30 vs. No. l. The result of such presettime-phase displacement yields in summation, output signals 75 ofoptimum. stepped waveform construction as willbe detailed hereinafter,particularly in connection with FIGS.-4 to 7.

Reference summing transformer ST-I is shown with both its primary 6-1and secondary 65-1 connected in wye configuration. The central or commonterminal 69 of wye secondary 6S affords'the neutral wire connection (N)for the three-phase four-wire output indicated at 75. Cable 70 connectsthe secondary windings of the summing transformers in particularseries-add, as shown in FIG. 2, and explained in connection with FIGS. 7and 10 for the two-channel version hereof. Balanced polyphase outputsignal summation occurs thereby, producing optimum stepped waveforms ofrelatively low distortion that are readily filtered'etfectively.

The square wave power fed by generator 55 -to primary 60-1 of summingtransformer ST-l is graphically illustrated in FIG. 4. Each primarywinding 62-1,63-l,64-1 is impressed by a square waveform voltage ofalternate positive and negative components, each of duration at thefrequency of system output, namely 1",. With winding 62-1 as referencephase (11,; winding 63-] is at phase impressed with square wave voltagethat lags in time-phase by I20, as shown; and winding 64-1 is at phase45 impressed with voltage that lags 4), by 240, and d), by l20. Thevoltage induction to secondary 65-1 is on an effective square wavebasis, as will now be described.

Apparatus for the generation of three-phase square wave power, per FIG.4, may embody banks of transistors, siliconcontrolled rectifiers, orgas-filled tubes. Exemplary SCR circuitry therefor is schematicallyshown in FIG. 3, and in more detail in my above-said patent. Theinvention inverter system requires an even number of pairs of suchpolyphase square voltage generators (as 40), with respectivepredetermined time-phase displacement between said channels forenergizing the corresponding channels thereof. Thus, the two-channelembodiment 40 utilizes polyphase square wave power via cable 58 ofpaired generator 56 that is displacedby 30 in time-phase with respect tothe reference three-phase counterpart from generator 55, for reasons tobe set forth in connection with FIG. 7. Circuits for obtaining therequisite precise timing, triggering and commutating the SCR thereof toprovide the sequential input voltages are detailed in the said patent,and not part of this invention.

FIGS. 5 and 6 illustrate the resultant waveforms of the signals thatappear at the secondary (65-1) output of transfonner ST-l when theaforesaid three-phase square wave voltages are applied to its primary(60-1) input. Induced in each channel No. 1 secondary windings66-1,67-1,68-1 is a stepped waveform voltage. FIG. 5 illustrates itswaveform output, at line-to-neutral-point 69. That across winding 66-1(phase 1 1) is in time-phase with the 5, square wave input (FIG. 4),extending with and swings, each 180 long. The secondary phases 4: and 41are correspondingly 120 and 240 time-phase displaced, as will now beunderstood by those skilled in the art. Each step" of the FIG. 5 wavesis 60 long, and 0.5 in relative magnitude Each such full wave over 360is thus in eight steps, corresponding to one cycle for the f, output.The plateau peaks are at the relative magnitude 1.0 for a the purpose ofthe subject presentation.

Such stepped output waveform results from the voltages induced by thepolyphase transformer (ST-1) induction, due to the three-phase squarewave input thereto (see FIG. 4). The corresponding line-to-linewaveforms, as across lines (I), to 4: are shown in FIG. 6; its timereference also being that of the phase ii), input per FIG. 4. It isnoted that this signaloutput (rb, to 11: is only of 120 duration forboth its and sections, with 60 blanks or zero current and voltagetherebetween. Further, its relative amplitude is 0.866 compared to thereference line-to-neutral waveform (FIG. The waveform of the signaloutput between lines (11 and 4);, is the same as between lines 4), to4),, except it lags 120 to coincide with input phase rb that betweenlines 41 and being further behind by 120.

It is understood that the polyphase square wave input voltages to thepolyphase primaries of the summing transformers hereof (ST-l, ST-2)correspond to the idealized waveforms of FIG. 4. Although sharp risesquare waveforms are illustrated, the invention system is also effectivewith input waveforms having rounded corners or with sloped rise-and-fallshape. Particular means for generating such polyphase square wavecurrents are optional, in utilizing the invention system. Thethree-phase square wave signal array shown in FIG. 4 corresponds toe.m.f. measurements from each output line (of cable 57) to a virtual orvoltage midpoint of the generator (55), or to the opposite link duringconduction, as is understood by those skilled in the art. Theline-to-line voltage for secondary 65-1 is as shown in FIG. 6 on athree-phase basis however. 1 The magnitude of the secondary voltagesdepends upon the tums-ratio, primary (60-1) to secondary (65-), as willbe set forth. Finally, the corresponding parameters of the polyphaseoutputs of the other square wave generator(s) in the system, are intime-phase displacement with respect to the base generator (55), asalready indicated.

A typical polyphase square waveform generator (55) electrical circuit isschematically indicated in FIG. 3. Its threephase voltage output atterminals 57 corresponds to the conduction curves of FIG. 4. Thealternate firing of paired siliconcontrolled power rectifiers -1, 82-1,l80 intervals, produces the output phase that of SCR 80-2, 82-2: phase#2 of SCR 80-3, 82-3: phase da Their respective inductors (as 84-1,86-1) in series to the DC bus 47, 48 are part of their commutationcircuitry, and also serve to moderate or soften the rate of reappliedvoltage and current to the power SCR. The shunt diodes -1, 87-1 etc.,are for conducting reactive load currents back to the DC bus, as is wellknown. The respective circuitry for the cyclic gating ON and commutatingor turning OFF of the SCR pairs 80-1, 82-1 etc., to produce their squarewaveform outputs per F IG.-4 are applied at 90-1, 90-2, 90-3. Suchcircuitry and their logic signal control are set forth in the saidpatent and in my copending patent application Ser. No. 868,191 filedOct. 21, 1969 for Commutation Circuit for Power Inverters," assigned tothe assignee hereof.

FIGS. 7A to 7D illustrate the principles of the system output waveformconstruction for the two-channel inverter (40)..

FIG. 7A corresponds to FIGS. 2 and 5, depicting reference summingtransfonner ST-I with an applied line-to-line square wave voltage E(three-phase) to its wye connected primary windings (60-). TransformerST-l connects to the output 57 of three-phase square wave generator(55). The line to neutral secondary waveshape (e,), for the referencewinding at 0 (and on a three-phase basis, with the other windings) isthe eight-step waveform of FIG. 5 The primary (62-1) to respectivesecondary (66-1) winding ratio results in waveshape (c with a peakmagnitude b indicated as at unity level 1 .0, with its intermediatesteps a at the 0.5 level. The secondary windings (65-1) of transformerST-l are preferably wound close onto their respective phase primarywindings (60-1) for efficient results. They are also in wye-connectionas shown.

FIG. 7B shows the operation of the second summing transformer (ST-2) ofthe two-channel inverter (40). Its primary winding60-2 is three-phaseand is delta-connected to the three-phase square wave output (58) ofgenerator SCR-2 (56) as shown in FIG. 2. The generator 56 signals aredisplaced 30 in time phase-by-phase behind those of generator 55,'asshown in FIG. 8. The resultant and waveform (e of the respective ST-2secondary windings 65-2, correspond to that shown in FIG. 6 forline-to-line of the ST-1 secondary windings 65-1. The resultant waveform2 has 60 alternating blanks, and a relative magnitude of 0.866 for (c),for its and sections. The said square wave voltage e depicted in FIG. 73corresponds to e, of the phase (I), secondary coil 66-1 of wyetransfonner ST-l. Voltage e is that across secondary winding 66-2 ofdelta transformer ST-2, as shown. The corresponding voltages of theirand 4);, phases are respectively 120 and 240 therefrom, as will beunderstood. The significant phase relationship between the voltages e,and eof each of the phases is that the 120 long waveforms of e, start 30behind the reference 0 (and 180) of e,, as illustrated.

The predetermined respective time-phase displacement between the e, ande secondary voltages is that generated between the outputs of squarewaveform units 55, 56. As said units are triggered by a common circuitunit (54) and clock (45), such time phasedisplacement is readily set upand precisely maintained as explained in my aforesaid system patent.Corresponding secondary windings are connected in voltage series-add(e,+ relation, phase-by-phase, as indicated in FIG. 7C, as well as inFIG. 2. The exemplary connections are from secondary neutral point n, ofST-l, in series-add of the respective secondary windings of 60-1, 60-2,to yield resultant (e;,) outputs in the three phases. Similarconnections are made for the three secondary winding sets oftransformers ST-l and ST-2, which with the neutral n, lead (N) providethe four-wire output 75, as will be understood. Alternatively, theneutral (m) may be arranged with the three delta channel No. 2secondaries 62-2, 63-2, 64-2 in wye-connection, and their wye channelNo. l counterparts seriesadded thereto.

Their respective output voltage resultants (e illustrated on aninstantaneous real-time basis, are the l2-step waveform of well-knowneffectiveness for inverter output circuits. In threeor six-phase systemssuch square stepped waveforms require relatively even less filtering.The optimum l2-step waveshape of output signals e each extend across 360in alternate and sections 180 each. Their peaks at d are at relativemagnitude 1.866, combining the b and c levels of signal components 2 ande The peak plateaus each extend for 60 in time, the intermediate stepsbeing 30Such l2-step waveshape (a is synthesized herein in a stable,simple and direct manner in two channels with two output transformersST-l, ST-2 of conventional polyphase construction. Correspondingwaveforms with significantly larger numbers-of steps can be produceddirectly, with even greater advantage, in versions hereof with morechannels, as will be set forth.

Waveform e is thus at each output phase of the two-channel inverter(40), each being 120 apart in time-phase. lnterconnection of theirsecondary windings, in three-phase array with line neutral (N), isdiagrammed in FIG. 2. Output cable 75 connects to the three-phase filter(42) (see FlG. 1) that removes the harmonic content of the initialstepped waveforms, providing clean polyphase output currents at thesystem frequency (f,,). FIG. 7D illustrates the resultant output insinusoidal waveform e,,, for each phase. Said 12-step waveforms have noharmonics below thell 1th when so combined, and in three-phasearrangement, do contain the llth and 13th harmonic of fundamental f,,.In a balanced threephase system the next harmonics that appear are the23rd and 25th second order in magnitude. The latter are inexpensivelyfiltered, for fundamental f, frequencies of 400 hertz or higher.Relatively small series resonant filters effectively strip out the 1 lthand 13th harmonic content. Higher harmonics, and radio frequencyinterference, are readily removed with small series chokes and parallelcapacitors. The filtering unit (42) for the polyphase inverters hereofis significantly less in weight and cost, for a given power output, ascompared to those required for other types of inverters orcycloinverters.

An important advantage of such series'arrangement of the output windingsis that for a rated output voltage, the requisite voltage from eachtransformer (ST-l, ST-2) need only be about one-half the final one. Thisresults in less costly SCRF for the generators (55, 56). ln effect, theresult is current multiplication, for any given-output power rating. Theuse of multichannels for the inverters hereof is economical for higherpower inverter systems, as described in my aforesaid system patent.

As already mentioned, the winding construction of the respectivethree-phase transformers ST-l and ST-2 is to have the same impedance ormatch to their respective square wave generators 55 and 56. This assumesthat the 55, 56 units have the same rating, output voltage and currentcharacteristics, waveform and impedance. A significant difference is thenormally preset time-phase displacement between their respective outputphases, as 30 for a two-channel system 40, as explained hereinabove inconnection with FIGS. 2, 7B and 8. The interline impedances are the sameat outputs 57, 58: 2, between phases di -4);; Z at di -(b 2,, at Thenumber of winding turns (N used in each primary winding 62-1, 63-1, 64-1of transformer ST-l is selected to provide said matched impedance intheir wye-connected array. To provide the same impedance matching withgenerator 56, the delta connected primary windings 62-2, 63-2, 64-2 eachhaves/511,, turns. In this manner both generators 55 and 56 are moreequally loaded with power demand by their respective transformers ST-land ST-2 in the operation of inverter 40 with a load across output lines(75). The SCR' of the system generators (55, 56) thereby share in outputcurrent demand substantially equally during normal, overloaded,unbalanced, and

even reactive load conditions.

In further balancing of the loading between the generators 55, 56, theefiective output voltage contribution from each secondary winding ofboth transformers ST-l, ST-2 are made to be substantially equal. Towardsthis end, their number of winding turns (n,,) are made the same, for theconfiguration hereinabove set forth for their respective primarywindings. Thus, if n, of the secondaries 66-1, 67-1, 68-] is made equalto that (n,,) of their primary windings, then that for the 65-2secondaries is n as well, wherein their primary windings (60-2) eachhave #511,, turns as aforesaid. Such respective ratios result in therelative 0.5, 1.0, 0.866, and L866 magnitudes for signal levels a, b, c,d in FIG. 7, and provides the 12-step waveform e The waveforms per e,and e, have the same effective secondary R.M.S. output voltage.

In practice, with line-to-line voltages (5,) across generator terminals57 and 58 at 200 volts, the line-to-neutral equivalent is 1 l5 volts.With n',,=n,,, the effective e voltage is l 15 volts, as it that for eTheir composite series-add voltage e;, is twice that, an R.M.S. voltageof 230 line-to-neutral (N) at inverter (40) output terminals 75. If aline-to-neutral voltage is instead desired at this output (75), theturns n, of the transformer secondaries are made half of n,,-, resultingin a 200 volt line-to-line level, as will be understood. The invertersystem 40 hereof has these desirable characteristics: stable; balancedloading between the generators 55, 56; line voltage maintenance (75);little harmonic content with inexpensive effective filtering (42);longer life-periods and lower rated SCR' for given system output powerrating; less cost and weight for the transformers and filter.

As mentioned hereinabove, more than the two channels per inverter 40 areadvantageously employed for higher output power ratings. The powercontribution and design-load of each generator-channel is the systemoutput rating divided by the number of channels used, see aforesaidinverter system patent. The operational interchannel principles of theinvention hereof are applicable to an even number of channels (n) as 2,4, 6 2n. Where four channels are used, the respective time-phasedisplacement of the square wave output of channel No. 2 with respect tothat of channel No. l is l5 of channel No. 3, 30; of channel No. 4, 45'.For a six-channel inverter system, such respective time-phasedisplacements are l0, 20, 30, 40, 50. Their delta and wye transformerconnections may be arranged in pairs, in succession. Their resultantwaveforms have a correspondingly greater number of steps, thus requiringan even smaller filter structure than for a two-channel system ofequivalent power rating.

The exemplary output summing transformers (ST-l, ST-2) for each channelwere stated to be of conventional polyphase design and construction.This is an important advantage in that their production, weight, andcost factors may be directly controlled. No exotic material or toughdesign features are required herein. Thus, conventional powertransformer laminations for the system frequency can be used. Theinterphase magnetic iron coupling and its structure in general isoptional; efficient polyphase magnetic interaction being preferred. Thewidely used flat open-window" E-l arrangement has been found to bepractical and economical herein. Other three-phase transformerconfigurations may be used, as: those shown in US. Pat. No. 2,616,070;three C-cores joined at their open ends into a three-dimensional Y"form; three parallel l-cores with their ends joined by laminatedcrossbars. Respective primary and secondary windings are wound on eachleg of the three phasesQThe relative turns and tum-ratios for the wyeand the delta channel transformers were outlined hereinabove.

What is claimed:

1. An alternating current power supply system comprising a plurality ofstatic polyphase inverters energizable by a unidirectional currentsource, each of said inverters providing an individual polyphase set ofalternating signal trains of generally square waveform that are insymmetrical time-phase displacement within each signal set, means formaintaining the said polyphase signal sets at a predetermined commonfrequency and at substantially the same signal shape, means forestablishing a time-phase displacement respectively between successiveof said polyphase signal sets, a polyphase output transformer individualto each of said polyphase inverters, each of said transformerseffectively having primary windings in circuit connection with theoutput of its associated inverter, the primary windings of at least oneof said transformers being in Wye-connection, those of at least a secondof said transformers being in delta-connection, an individual secondarywinding magnetically coupled witheach of the said transformer primarywindings, and circuit means connecting in series-add relation respectivesecondary windings that contain related signal trains among thepolyphase output signal sets to form separate output-phase groups thatestablish a common polyphase power output for the alternating currentsystem. I

2. A power supply system as claimed in claim 1, containing only twopolyphase signal inverters, the output of one of said inverters beingconnected with said Wye-connected transformer, the other inverter outputbeing connected with said delta-connected transformer, and therespective signal sets of the two inverters being in time-phasedisplacement in the order of 30?.

3. A power supply system as claimed in claim 1, in which the respectiveprimary windings of said deltaand Wye-connected transformers areproportioned to present an effective line-toline impedance thatsubstantially matches that between the output lines of their respectiveconnected polyphase inverter.

4. A power supply system as claimed in claim 2, in which the respectiveprimary windings of said deltaand Wye-connected transformers areproportioned to present the same effective line-to-line impedance, andwhich impedance substantially matches that between the output lines oftheir respective connected polyphase inverter.

-5. An alternating current power supply system comprising a plurality ofstatic three-phase inverters energizable by a unidirectional currentsource, each of said inverters providing an individual set ofthree-phase alternating signal trains of generally square waveform thatare in symmetrical time-phase displacement within each signal set, meansfor maintaining the said three-phase signal sets a predetermin d commonfrequency and at substantially the same signal shape, means forestablishing a predetermined time-phase displacement respectivelybetween successive of said three-phase signal sets, a three-phase outputtransformer individual to each of said inverters, each of saidtransformers effectively having primary windings in circuit connectionwith the output of its associated inverter, the primary windings of atleast one of said transformers being in Wye-connection, those of atleast a second of said transformers being in delta-connection, anindividual secondary winding magnetically coupled with each of saidtransformer primary windings, and circuit means connecting in series-addrelation respective secondary windings that contain related signaltrains among the three-phase output signal sets to form separateoutput-phase groups that establish a common three-phase power output forthe alternating current system.

6. A power supply system as claimed in claim 5, containing twothree-phase signal inverters, the output of one of said inverters beingconnected with said Wye-connected transformer, the other inverter outputbeing connected with said deltaconnected transformer, and the respectivesignal sets of the two inverters being in time-phase displacement in theorder of 30.

7. A power supply system as claimed in claim 5, in which the respectiveprimary windings of said deltaand Wye-connected transformers areproportioned to present an effective line-toline impedance thatsubstantially matches that between the output lines of their respectiveconnected three-phase inverter.

8. A power supply system as claimed in claim 6, in which the respectiveprimary windings of said deltaand wyeconnected transformers areroportioned to resent the same efiective line-to-llne rmpe ance, andwhlc impedance substantially matches that between the output lines oftheir respective connected three-phase inverter.

9. A power supply system as claimed in claim 8, wherein both saidinverters have substantially the same power rating and line-to-lineoutput impedance, and each said primary winding of the delta-connectedthree-phase transformer has V times the number of turns of each saidprimary winding of the Wye-connected transformer.

10. A power supply system as claimed in claim 8, in which saidtransformers are wound with respective primary to secondary winding turnratios that provide secondary winding output voltages havingsubstantially the same root-mean-square voltage level, whereby each saidseries-add output-phase group has its windings contribute powerequivalently.

11. A power supply system as claimed in claim 9', wherein each of thetransformer secondary windings have the same number of turns and therebysubstantially the same rootmean-square output voltage level, wherebyboth said inverters equally divide the output power demands on thesystem.

1. An alternating current power supply system comprising a plurality ofstatic polyphase inverters energizable by a unidirectional currentsource, each of said inverters providing an individual polyphase set ofalternating signal trains of generally square waveform that are insymmetrical time-phase displacement within each signal set, means formaintaining the said polyphase signal sets at a predetermined commonfrequency and at substantially the same signal shape, means forestablishing a time-phase displacement respectively between successiveof said polyphase signal sets, a polyphase output transformer individualto each of said polyphase inverters, each of said transformerseffectively having primary windings in circuit connection with theoutput of its associated inverter, the primary windings of at least oneof said transformers being in wye-connection, those of at least a secondof said transformers being in delta-connection, an individual secondarywinding magnetically coupled with each of the said transformer primarywindings, and circuit means connecting in series-add relation respectivesecondary windings that contain related signal trains among thepolyphase output signal sets to form separate output-phase groups thatestablish a common polyphase power output for the alternating currentsystem.
 2. A power supply system as claimed in claim 1, containing onlytwo polyphase signal inverters, the output of one of said invertersbeing connected with said wye-connected transformer, the other inverteroutput being connected with said delta-connected transformer, and therespective signal sets of the two inverters being in time-phasedisplacement in the order of 30*.
 3. A power supply system as claimed inclaim 1, in which the respective primary windings of said delta- andwye-connected transformers are proportioned to present an effectiveline-to-line impedance that substantially matches that between theoutput lines of their respective connected polyphase inverter.
 4. Apower supply system as claimed in claim 2, in which the respectiveprimary windings of said delta- and wye-connected transformers areproportioned to present the same effective line-to-line impedance, andwhich impedance substantially matches that between the output lines oftheir respective connected polyphase inverter.
 5. An alternating currentpower supply system comprising a plurality of static three-phaseinverters energizable by a unidirectional current source, each of saidinverters providing an individual set of three-phase alternating signaltrains of generally square waveform that are in symmetrical time-phasedisplacement within each signal set, means for maintaining the saidthree-phase signal sets a predetermined common frequency and atsubstantially the same signal shape, means for establishing apredetermined time-phase displacement respectively between successive ofsaid three-phase signal sets, a three-phase output transformerindividual to each of said inverters, each of said transformerseffectively having primary windings in circuit connection with theoutput of its associated inverter, the primary windings of at least oneof said transformers being in wye-connection, those of at least a secondof said transformers being in delta-connection, an individual secondarywinding magnetically coupled with each of said transFormer primarywindings, and circuit means connecting in series-add relation respectivesecondary windings that contain related signal trains among thethree-phase output signal sets to form separate output-phase groups thatestablish a common three-phase power output for the alternating currentsystem.
 6. A power supply system as claimed in claim 5, containing twothree-phase signal inverters, the output of one of said inverters beingconnected with said wye-connected transformer, the other inverter outputbeing connected with said delta-connected transformer, and therespective signal sets of the two inverters being in time-phasedisplacement in the order of 30*.
 7. A power supply system as claimed inclaim 5, in which the respective primary windings of said delta- andwye-connected transformers are proportioned to present an effectiveline-to-line impedance that substantially matches that between theoutput lines of their respective connected three-phase inverter.
 8. Apower supply system as claimed in claim 6, in which the respectiveprimary windings of said delta- and wye-connected transformers areproportioned to present the same effective line-to-line impedance, andwhich impedance substantially matches that between the output lines oftheir respective connected three-phase inverter.
 9. A power supplysystem as claimed in claim 8, wherein both said inverters havesubstantially the same power rating and line-to-line output impedance,and each said primary winding of the delta-connected three-phasetransformer has 3 times the number of turns of each said primary windingof the wye-connected transformer.
 10. A power supply system as claimedin claim 8, in which said transformers are wound with respective primaryto secondary winding turn ratios that provide secondary winding outputvoltages having substantially the same root-mean-square voltage level,whereby each said series-add output-phase group has its windingscontribute power equivalently.
 11. A power supply system as claimed inclaim 9, wherein each of the transformer secondary windings have thesame number of turns and thereby substantially the same root-mean-squareoutput voltage level, whereby both said inverters equally divide theoutput power demands on the system.